@vectorize_2arg should use broadcast

It seems like the `@vectorize_2arg` macro should really be broadcasting.

e.g. I expected this to work and be equivalent to `broadcast(atan2, y, x)`:

```
y = linspace(-1,1,50)
x = y'
theta = atan2(y, x)
```

but it gives `ERROR: argument dimensions must match`.
